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266                               CHEMICAL ENGINEERING
fragments or particles are all of the same composition the machine will effect a grading. The function of the air blown through the cloth is to keep the bed loose so that there will be no packing thus permitting interstitial action to have full play. It has been stated that the air actually raises and holds in a state of balance the larger particles. It is true that with fine materials the air creates so much looseness in the bed that the upper layers appear to be in a state of suspension and with closely sized material such a conception of the principle of operation may have some modicum of truth in it.
But it is really a superficial and unnecessary notion of the principle of operation. In the case of the middlings purifiers there is actual raising of some of the small light particles from the lower portions of the bed to the top by aid of the air current but by far the larger part of the light material is brought to the surface of the bed by interstitial action and is aspirated out of the machine from this point by the action of the fan. Aspiration is not an effect desired on the Sutton-Steele table. It will be evident on consulting the concentration section of this work that since large particles tend in a fluid or current to fall faster than small the governing principle of separation cannot be an air current. In the next described machine suitable for material of a non-packing character, separation into layers of different sized particles takes place entirely by interstitial action and without aid of air currents at all.
Separation by Interstitial Action and Deflection.—Machines of this type are largely used in oatmeal factories for separating the groats (hulled oats) from - the annulled oats. They are also recommended for separating hulled from unhulled or paddy rice, garlic from wheat, and stone from wheat. The last application seems somewhat dubious. The apparatus consists of a shaking frame on which is mounted one or more fiat decks. An actuating mechanism shakes the decks back and forth and there are means for changing the slope of the decks and a speed-changing device so that the number of throws per minute can be altered while the machine is running. The decks are separated into 6 to 12 compartments each facing openings in the feed box placed along the upper sides of the decks. The compartments on the decks are defined by triangular deflectors of steel plate the sides of the deflectors'being at right angles to the plane of the deck. On groats and hulled oats being fed to the decks the greater slipperiness of the latter and their smaller size allows them to settle through the bed of mixed material on the decks. Owing to the transverse slope of the decks the lower layers consisting largely of the hulled oats will work down to the lower side of the deck where they are discharged. The groats in the upper layers partake more nearly of the back and forth motion of the decks and meeting the deflectors are carried up the deck contrariwise to the direction of travel of the hulled oats. The groats travel toward the upper or feed side of the machine and discharge over a dam or baffle placed along that side of the decks.
In the case of separation of garlic from wheat the garlic would follow the line of travel of the groats because of its greater size. It is probable that in all cases of separation on this device slight differences in density assist the separatory actions (see page 243 et seq.}.
Separation Depending upon Sliding Friction.—Two devices both from the anthracite-coal industry will illustrate the application of this principle of separation. According to Ayres, the inventor of the separator shown in Fig. 3, anthracite coal may be grouped into the following classes:
1.  Glassy fracture coal, usually cubical in form.
2.  Flat coal, some pieces having slate faces.